a hole in the wall

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in: "But there was a problem with that monitor. He didn't know a Processor Queue Length from a hole in the wall."

It's about a computer monitor displaying data for the monitoring of a computer network. I do not understand what 'hole in the wall' means here. According to Answers.com 'hole in the wall' means : A small, modest, or obscure place... so what's the relation with a computer network ?

Yes, Jonathan's idea is right; and it's important that whatever takes the place of the 'hole-in-the-wall' bears no similarity whatsoever to the main object. cf. Gilbert & Sullivan's Major General, proud to be able to "tell a rifle from a javelin"!